1. Field of the Invention
The present invention relates to an index guided semiconductor laser device and a method of manufacturing the same.
2. Description of the Related Art
In recent years, the development of a semiconductor laser device has remarkably advanced. For example, a conventional He-Ne gas laser can be replaced with an InGaAlP laser, and the InGaAlP laser can be used in various applications. The InGaAlP laser attracts a great deal of attention as a key device for obtaining high performance of each equipment. Under these circumstances, an index guided semiconductor laser device is used as a light source of an optical disk system, a laser printer, or the like.
A conventional SBR (Selectively Buried Ridge Waveguide) laser will be described below with reference to FIGS. 1 to 3 showing an ridge-stripe type SBR laser.
FIG. 1 is a sectional view showing a laser device of the first prior art. In FIG. 1, reference numeral 1 denotes an n-type GaAs semiconductor substrate; 2, a first cladding layer; 3, In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P active layer; 4, a second cladding layer; 5, a stripe-like ridge formed on a portion of the upper surface of the second cladding layer 4; 6, a current blocking layer formed on the second cladding layer 4 except for the upper surface of the ridge 5; 7, an intermediate layer formed on the upper surface of the ridge 5; 8, a contact layer; and 9 and 10, electrodes. The steps in manufacturing the laser device are described as follows. The above layers were sequentially and continuously and epitaxially grown by a low-pressure MOCVD method using a source gas having the elements for constituting the above layers as their compositions. A portion of the upper surface of the second cladding layer 4 was chemically etched to form the stripe-like ridge 5 on the portion of the upper surface of the second cladding layer 4. Thereafter, the current blocking layer 6 was formed on an etched portion of the second cladding layer 4, and the contact layer 8 was grown on the current blocking layer 6 and the intermediate layer 7, such that the electrodes 9 and 10 were formed on the upper surface of the contact layer 8 and the lower surface of the semiconductor substrate 1, respectively.
In the steps of etching the upper surface of the second cladding layer 4 and forming the stripe-like ridge 5 at a portion of the second cladding layer 4, since the area of the etched portion is considerably larger than that of the remaining portion, the dimensions such as the height and width of the ridge 5 cannot be easily controlled with high precision. This is because a new etchant cannot be exchanged for an old etchant smoothly on the surface of the crystal becauses of a large etching area. That is, since the upper surface of the second square cladding layer 4 having a side of 300 .mu.m is etched to leave the ridge 5 having a width of about 5 .mu.m and a height of about 0.8 to 1.0 .mu.m, the dimensions of the ridge are varied within a range of about 10 to 20%.
Upon etching the second cladding layer, when the current blocking layer 6 is to be formed on the upper surface of the second cladding layer 4 to bury the ridge 5, a crystal is not easily grown near the ridge 5 due to the large height of the ridge 5. For this reason, a crystal which can be grown to a level allowed in terms of characteristics is limited to GaAs. However, when GaAs is subjected to epitaxially growth, since the height of the ridge 5 is too large, the GaAs is grown in different plane directions near the ridge 5. In this portion, crystal defects of the current blocking layer 6 easily occur.
In addition, in the laser having the above arrangement, laser characteristics such as a threshold current, temperature characteristics, or astigmatism are influenced by the width of the ridge 5 and the thickness of the etched portion of the second cladding layer 4. Therefore, the laser characteristics and the distribution of the laser characteristics in the wafer surface are varied by variations in width of the ridge 5 and thickness of the second cladding layer 4 upon the etching operation, and a transverse mode cannot easily be controlled. Furthermore, in the portion in which the ridge 5 is buried, i.e., in the current blocking layer 6 near the ridge 5, crystallinity of this portion largely influences reliability of the laser. Therefore, in this laser device, crystal defects are easily concentrated, and a failure rate is easily increased to cause degradation of the reliability of the laser device.
Since the current blocking layer 6 must be made of GaAs, the band gap of the active layer 3 is larger than that of the current blocking layer 6, and the structure of this laser is limited to a loss guided structure. Therefore, characteristic improvements, e.g., a decrease in threshold current, improvement of temperature characteristics, or a decrease in astigmatism are limited.
FIG. 2 is a sectional view showing a laser device according to the second prior art. In this prior art, unlike in the above first prior art, the laser device is arranged to have an arrangement which can withstand against optical feedback noise to be applied to an optical disk system. In FIG. 2, reference numeral 11 denotes an n-type GaAs semiconductor substrate; 12, a first cladding layer; 13, an In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P active layer; 14, a second cladding layer; 15, a stripe-like ridge formed on a portion of the upper surface of the second cladding layer 14; 16, a current blocking layer formed on the second cladding layer 14 to leave a stripe-like upper intermediate portion of the ridge 15 in the longitudinal direction; 17, an intermediate layer formed on the upper surface of the ridge 5; 19, a contact layer; and 20 and 21, electrodes. Note that a conductive region 22 is formed by the stripe-like portion left on the upper intermediate portion of the ridge 15. The steps in manufacturing the above laser will be described below. As in the first prior art, the above layers were continuously grown on the upper surface of the semiconductor substrate 11, and the upper surface of the second cladding layer 14 was etched to form the stripe-like ridge 15 on a portion of the upper surface of the second cladding layer 14. Thereafter, the current blocking layer 16 was grown on the second cladding layer 14 to leave a stripe-like upper intermediate portion of the ridge 15 in the longitudinal direction. That is, the current blocking layer 16 was formed on the edge portions of the ridge 15 along the longitudinal direction and on the etched portion of the second cladding layer 14. In addition, the contact layer 19 were grown, and the electrodes 20 and 21 were formed on the upper surface of the contact layer 19 and the lower surface of the semiconductor substrate 11, respectively.
In the above steps in manufacturing the conventional laser device, as in the first prior art, in the step of forming the ridge 15, since the area of a portion to be etched is considerably larger than that of the remaining portion, the dimensions such as the height and width of the ridge 15 cannot be easily controlled with high precision. In addition, when the current blocking layer 16 is formed on the upper surface of the second cladding layer 14 upon the etching operation to bury the edge portions of the ridge 15 so as to form the conductive region 22, the height of the ridge 15 is high, i.e., 0.5 .mu.m or more, and a crystal is not easily grown near the ridge 15. Therefore, a crystal which can be grown to a level allowed in terms of characteristics is limited to GaAs. In this case, crystal defects are easily concentrated on the crystal of the current blocking layer 16 near the ridge 15.
In the laser having the above arrangement, laser characteristics such as a threshold current, temperature characteristics, or astigmatism are influenced by the width of the ridge 15, the thickness of the etched portion of the second cladding layer 14, and the width of the conductive region 22. Optical feedback noise characteristics are influenced by these dimensions. Therefore, the laser characteristics and the distribution of the laser characteristics in one wafer are varied by etching precision. The etching operation must be controlled with high precision to obtain desired laser characteristics. In addition, since crystal defects are easily concentrated on a portion of the current blocking layer 16 near the ridge 15, and a failure rate is easily increased, the reliability of the laser may be degraded.
Since the current blocking layer 16 must be made of GaAs, the band gap of the active layer 13 is larger than that of the current blocking layer 16. As in the first prior art, the structure of this laser is limited to a loss guided structure. Therefore, characteristic improvements, e.g., a decrease in threshold current of the laser characteristics, improvement of temperature characteristics, or a decrease in astigmatism are limited. In addition, a laser device having small optical feedback noise cannot be easily obtained at a high yield with excellent reproducibility.
FIG. 3 is a sectional view showing a laser device according to the third prior art. In this prior art, as in the second prior art, the laser is arranged to have an arrangement which can withstand against optical feedback noise. However, unlike the laser device of the second prior art, in the laser device of the third prior art, a laser device having a short oscillation wavelength is used to be applied to an optical disk having a higher recording density. In FIG. 3, reference numeral 111 denotes an n-type GaAs semiconductor substrate, the plane direction of the upper surface of the semiconductor substrate is inclined from directions (0,0,1) to [1,1,0] by, e.g., 15.degree. C. Reference numeral 112 denotes a second cladding layer; 113, an In.sub.0.5 (Ga.sub.1-x Al.sub.x)0.5P activelayer; 114, a second cladding layer; 115, a stripe-like ridge formed on a portion of the upper surface of the second cladding layer 114; 116, a current blocking layer formed on the second cladding layer 114 to leave a stripe-like upper intermediate portion of the ridge 115 in the longitudinal direction; 117, an intermediate layer formed on the upper surface of the ridge 115; 119, a contact layer; and 120 and 121, electrodes. Note that a conductive region 122 is formed by the stripe-like portion left on the upper intermediate portion of the ridge 115. In the steps in manufacturing the above laser, the above layers were continuously grown on the upper surface of the semiconductor substrate 111 having a plane direction inclined from directions (0,0,1) to [1,1,0] in the same manner as described in the second prior art, and the stripe-like ridge 115 and the like were formed.
In this prior art, in addition to the same problems as described in the second prior art, the following problem is posed. When the ridge 115 having a large height is formed by an etching operation using, e.g., a phosphoric acid enchant, in the step of forming the ridge 115, the ridge 115 has an asymmetrical shape in the right-and-left direction because the semiconductor substrate 111 is inclined. For this reason, since refractive index distribution having a direction parallel to the active layer 113 becomes asymmetrical, a transverse mode is set to be unstable by an increase in optical output, i.e., an increase in injection current. As a result, a characteristic curve representing current vs. optical output characteristics is kinked, and a far field pattern (FFP) is changed.